1. Field of the Invention
The present invention relates to IR-reflective elements made of impact-resistant plastic, and a process for their production.
2. Discussion of the Background
The use of disks of polymethyl methacrylate containing light-reflective particles aligned parallel to a surface is described in DE 25 44 245. The pigment particles used cause a selective reflection in the infrared range, which can be characterized by a selectivity number of greater than 1. TiO2, lead carbonate, and bismuth oxychloride are mentioned as IR-reflective pigments. The use of anatase-type TiO2 pigments, precipitated onto mica laminae, is described. Further, a layer of anatase-type TiO2 pigment having a thickness of approximately 120 μm is emphasized as being particularly advantageous.
The particles as described in DE 25 44 245 are dispersed in a partially polymerized methyl methacrylate to form a suspension at a concentration ranging from 0.01 to 1 wt. %. Thereafter, the suspension is fully polymerized, producing polymethyl methacrylate, in a chamber between glass plates. If the chamber is positioned horizontally, the pigment-mica particles of the suspension align parallel to the disk surface as they drop. This phenomenon yields fully polymerized disks having the desired IR-reflective effect. This parallel alignment of the IR-reflective particles can be improved if the glass plates of the chamber are moved several times in a circular motion relative to one another while the polymerizing material is still in a gel state.
DE 25 44 245 describes the possibility of working IR-reflective pigments into molding masses. The pigments then align parallel to the surface when the masses are processed using calendering, extrusion, or injection-molding.
EP-A 0 548 822 describes translucent, IR-reflective elements having a sun-protection and heat-insulation effect. These elements include sheets, web plates, and domelights. The elements have a transmission, T, in the visible spectrum ranging from 45 to 75%. The transmission is defined as the degree of light transmission for daylight, i.e. standard light type D65 or TD65. They also have a total energy transmission degree, g, ranging from 30 to 60%. Further, they have a ratio of T/g, i.e. TD65/g, at least 1.15. These measurements result from those experiments described according to DIN 67 507.
The translucent, IR-reflective elements of EP-A 0 548 822 can be produced from a rigid, amorphous base material made of translucent plastic and a translucent coating material containing from 20 to 40 wt. % of IR-reflective particles. The IR-reflective particles consist of a layer of titanium dioxide. The titanium layer is present at a thickness ranging from 60 to 120 nm on a lamellar-shaped carrier pigment, and is formed by co-extrusion or coating processes. Coating processes include lacquering or reverse-roll-coating. A coating layer that is 5 to 40 micrometers thick and contains the IR-reflective particles adheres to the base material made of a transparent binder not soluble in water. The IR-reflective particles are aligned parallel to the surface of the base material.
In the examples of EP-A 0 548 822, rutile type TiO2 pigments are used. EP-A 0 548 822 recommends the selection of a binder for the IR-reflective layer that has a lower melt viscosity than that of the base material for co-extrusion. In order to prevent the pigments from breaking, a twin-screw extruder having closely interlocking screws that rotate in opposite directions is used, and the pigment is worked directly into the melt.
Multi-web plates made of polymethyl methacrylate having a co-extruded IR-reflective finish are commercially available according to EP-A 0 548 822. Further, multi-web plates made of polycarbonate having a corresponding IR-reflective finish are known to improve weather-resistance. Here, another co-extruded layer containing UV-absorbers is located on the co-extruded pigment layer.
DE 196 569 A1 describes multi-layer interference pigments consisting of transparent carrier materials that are coated with alternating layers of metal oxides having a low and a high index of refraction, where the difference between the indices of refraction is at least 0.1. Accordingly, an alternating layer of TiO2/SiO2/TiO2 can be placed on mica laminae. The pigments described in DE 196 18 569 A1 are suitable for pigmentation of agricultural films in order to keep out infrared radiation from the sun, thereby preventing excessive heating of greenhouses.
EP-A 0 733 754 describes multi-web plates made of a polymethyl methacrylate modified to be impact-resistant. Depending on the impact resistance of the material used and the minimum thickness of the top flange of the multi-web plates, multi-web plates having high fracture resistance under stress due to hailstones is achieved. Accordingly, typical stress resistance of the multi-web plates in a hail shooting test using polyamide balls of at least 2 J, preferably at least 5 J, is desired and achieved. Furthermore, EP-A 0 733 754 additionally equips the multi-web plates with functional layers. These functional layers include scratch-resistance, anti-reflective, water-spreading or IR-reflective coatings on the outside or the inside of the multi-web plates.
JP-OS 08-53555 describes IR-reflective sheets of acrylic plastic containing an impact resistant modifier. The sheets are based on cross-linked emulsion polymers containing butyl acrylate and IR-reflective pigments in amounts ranging from 0.5 to 30 wt. %. The thickness of the sheets with co-extruded layers or for solid sheets may range 10 μm to 5 mm. Working the pigments into an acrylic plastic modified to be impact-resistant takes is achieved by utilizing two steps. First, the pigment is mixed with the granulate in the dry state. Then, the mixture is extruded to form a granulate.
JP-OS 08-52335 describes IR-reflective sheets or containing polycarbonate, and having a thickness of at least 0.5 mm. The sheets are also contain a co-extruded layer of an acrylic plastic modified to be impact-resistant. Further, the sheets contain IR-reflective pigments in amounts ranging from 0.5 to 20 wt. %. The co-extruded layers have thicknesses ranging from 20 to 300 μm. Working the pigments into the acrylic plastic modified to be impact-resistant is achieved by utilizing two steps. First, the pigment is mixed with the granulate while dry in a tumbler mixer. Then the mixture is extruded to form a granulate.
Multi-web plates with a high resistance to weathering and a very good resistance to hail impact are described in EP-A 0 733 754. For an additional IR-reflective finish on the multi-web plates, EP-A 0 733 754 recommends the application of a functional IR-reflective layer as described in EP-A 0 548 822. However, it has been shown that the additional pigment layer which imparts excellent IR-reflection and is co-extruded, is not as hail-resistant as a plate without the additional pigment layer. The IR-reflective layer, which can contain 25 wt. % IR-reflective pigment, is more brittle under stress due to hail compared to that of the underlying impact-resistant polymethyl methacrylate. This can result in the formation of tiny cracks in the IR-reflective layer in the presence of hail. These cracks are barely visible at first, but can propagate as the plate is subjected to the effects of weather. Therefore, the IR-reflective layer can start to flake off after some time.
DE 25 44 245 describes the possibility of working the IR-reflective pigments into molding masses. Here, the pigments align themselves parallel to a surface during the course of processing. However, it has been shown that it is not possible to achieve an effect that corresponds to EP-A 0 548 822. The alignment of the particles in a plastic element made of an extruded molding mass that contains uniformly distributed IR-reflective pigment is not as good as for a co-extruded or lacquered separate layer. Also, the relatively high tendency of these particles to fracture appears to be another problem. The lower rate of surface-parallel alignment, combined with a comparatively high proportion of breakage, generally leads to unsatisfactory results.
This is particularly true for plastic elements with a comparatively complex geometry deviating from a simple plate shape. During the production of such plastic elements, different melt flow directions and shear forces occur during extrusion which force the pigment particles into different alignments. Accordingly they are exposed to greater overall mechanical stress. These shapes include hollow elements such as double-web plates and, in particular, complicated hollow elements such as multi-web plates, latticework plates, etc.
JP-OS 08-53555 and JP-OS 08-52335 describe the extrusion and co-extrusion of acrylic plastic modified to be impact-resistant and containing IR-reflective pigments. Since the two components are mixed under dry conditions, a high proportion of the pigment breaks. Since the pigment fragments reduce transmission and reflect IR waves poorly or not at all, the efficiency of the IR-reflection cannot be optimized with respect to the amount of pigment used